The present invention relates to a plasma processing method and a plasma processing apparatus and, more specifically, to plasma etching used for a dry etching process that ionizes a source gas in a gas phase and processes the surface of a semiconductor material by physical or chemical reaction of highly activated particles of the plasma.
With the advance of miniaturization of semiconductor devices in recent years, there has been a growing tendency to form a wiring layer in multiple layers and to make the device structure three-dimensional. Under these circumstances, the fabrication of an isolation film used to keep wires and devices electrically isolated from one another has come to play an increasingly important role in the device manufacture. Etching of a silicon oxide film, the isolation film, has been done by using perfluorocarbon gas (PFC), such as CF4 and C2F8, and hydrofluorocarbon gas (HFC), such as CH2 and CHF3. This is because a carbon-containing gas is needed to cut off an Sixe2x80x94O bond of the silicon oxide film and generate a volatile compound.
As global environmental concerns are attracting growing attention, PFC and HFC are expected to be subjected to limited use or become difficult to obtain in the future because these gases easily absorb infrared rays, stay in atmosphere for as long as 3000 years and thus contribute greatly to the greenhouse effects on the earth.
The PFC and HFC gas plasmas contain fluorine, fluorocarbon radicals such as CF1, CF2 and CF3, and ions. An etching mechanism of a silicon oxide film operates as follows. These reactive species (e.g., radicals) stick to the surface of the silicon oxide film to be etched. The energy of ions incident on the surface gives rise to a localized quasi-high temperature condition, under which volatile products are formed by chemical reaction. Hence, to obtain good etching characteristics requires controlling the reactive species incident on a sample intended for etching and also controlling the energy and density of ions impinging on the sample. The control of the reactive species and of the density of ions in the plasma has been conducted by a plasma producing system in the etching equipment.
To generate reactive species in a reactor, Japanese Patent Laid-Open No. 74147/1995 for example discloses a method which involves forming the interior of the reactor using a carbon-based material and supplying carbon components into a plasma for etching.
Japanese Patent Laid-Open No. 363021/1992 describes making the reactor using ceramics to prevent degradation in the etching action of reactive species on the sample being etched and also discloses arranging a heater around the periphery of the reactor to alleviate plasma""s thermal shocks on the ceramics reactor.
When PFC and HFC gas plasmas are used, fluorocarbon- or carbon-based polymers adhere to the inner wall of the reaction chamber as the etching process of the sample proceeds. A method of removing the adhering polymers is known, which, as described in Japanese Patent Laid-Open No. 62936/1993, involves the installation of split, multiple electrodes-isolated from an outer wall of the reaction chamber-on the inner wall of the reaction chamber and the application of a radio frequency (RF) voltage between plasma generating electrodes successively to perform plasma cleaning. Further, Japanese Patent Laid-Open No. 231320/1989, 231321/1989 and 231322/1989 describe plasma cleaning methods which involve applying a voltage to electrodes electrically isolated with respect to the outer wall of the reaction chamber.
If such a conventional plasma cleaning is performed, there still will be particles adhering to the inner wall of the reaction chamber before the next cleaning operation. Because fluorine in the plasma reacts with the adhering layer on the inner wall of the reaction chamber, the fluorine density in the plasma decreases gradually, increasing the ratio of carbon in the plasma. That is, as a growing amount of particles adheres to the inner wall of the reaction chamber, the radical composition changes, causing a time-dependent change in etching characteristic, which poses a serious problem.
Etching equipment can be classified, according to the plasma producing system, into a capacitive coupling type, an ECR (electron cyclotron resonance) type, an ICP (induced coupling) type, and a surface wave excitation type. In the capacitive coupling type etching equipment, a material to be etched is placed on a bottom electrode and two voltage application systems apply differing frequencies and voltage to the upper electrode and the bottom electrode to control the plasma generation and the energy incident on the sample. The structure of this equipment, however, does not allow independent control of plasma generation and incident energy. The control of excited species in this equipment is considered to be performed by carbon or silicon used in the electrodes. However, no parameters on this control are available. Hence, it is necessary to perform three controls, i.e., control of the ion density, control of the energy of ions incident on the material being etched and control of reactive species, by controlling two, upper and lower, power supplies. Therefore, the range of parameters in which satisfactory etching characteristics can be obtained (defined as a process window) is narrow, making it difficult to produce stable etching conditions. The parameters that determine the etching characteristics include, in the plasma generation system, for example, RF power and microwave power applied between the electrodes, gas flow rate, gas pressure and gas composition. In the incident ion energy control, the etching characteristic determining parameters include the waveform and the frequency of the applied voltage and power.
In the plasma generation methods other than the capacitive coupling type, although the plasma generation control and the energy control of ions incident on the sample can be performed independently of each other, the mechanism for controlling the reactive species depends on the plasma generation control. Hence, these plasma generation methods have a drawback of having a narrow process window. In more detail, when a silicon oxide film of SAC (self-aligned contact) is processed in the high density plasma etching equipment, such as an ECR, there is a problem of a tradeoff between etch stopping at the bottom of holes and over-etching into a silicon nitride film. Further, the use of a high density plasma to perform a highly selective etching gives rise to another problem, a micro-loading phenomenon or RIE lag, in which the etching rate decreases as the hole diameters decrease, and an inverted micro-loading phenomenon or inverted RIE lag. Further, when metal films, such as TiN and Al laminated layers, are etched using this equipment, localized abnormal side-etched portions are formed (notching) at the boundary between different materials, such as TiN and Al.
Furthermore, with the method described in Japanese Patent Laid-Open No. 74147/1995, it is not possible to control the appropriate amount of excited species, making it difficult to perform an intended etching. The method disclosed in Japanese Patent Laid-Open No. 363021/1992 has a drawback of not being able to generate reactive species in the reactor.
The above problems can be solved by generating an exact amount of reactive species required for the etching in a region where the plasma comes into contact with the material to be etched.
This is detailed in the following. In the process of etching a silicon oxide film and a silicon nitride film on the sample to be processed, a gas containing fluorine, for example, is introduced into the reaction chamber, which is kept at a low gas pressure of 0.3 Pa to 200 Pa. An electric discharge is produced in the gas by applying an input power in the microwave and RF wave ranges to the gas to generate a plasma. Then, a solid material containing carbon, which is installed in the region where it contacts the plasma, has a DC or RF voltage applied thereto to release a required amount of carbon, thereby transforming fluorine radicals in the plasma into fluorocarbon radicals such as CF, CF2, CF3, CF4 for etching the material.
A gas containing fluorine, but not carbon, is introduced into the reaction chamber where the fluorine-containing gas reacts with the solid carbon allowing the silicon oxide film and silicon nitride film to be selectively etched without using PFC or HFC. That is, a plasma is produced from a fluorine gas not containing carbon and fluorine atom ions are made to react with solid carbon installed in the reaction chamber to produce compounds of carbon and fluorine, such as CF4, CF2, CF3 and C2F3. These compounds, radical molecules, have conventionally been able to be generated directly from dissociation of the PFC gas. These radical molecules thus generated have been used for etching the silicon oxide film.
The present invention is characterized in that reactive species required for etching the silicon oxide film and silicon nitride film are not supplied directly from PFC or HFC gas, but rather are generated from reaction with the solid carbon in the plasma chamber. This method makes it possible to generate reactive species necessary for etching so that the etching can be performed while maintaining selectivity as in the conventional process, even when the use of PFC and HFC gas is restricted or prohibited.
As for the improvement of selectivity and process margin during the process of making self-aligned contacts, this can be achieved by transforming reactive species into a single species of CF2, the etchant for the silicon oxide film. We have found that using carbon as the material for radical control and arranging it on the boundary surface with plasma can reduce the amount of fluorine in the plasma to one-half and increase CF1, CF2 and CF3 fivefold, tenfold and twofold, respectively, when compared to the case where aluminum is used as the radical control material. This is shown in FIG. 2, which illustrates the result of measurement of fluorine and CF2 in a CF4 plasma when Al, SiO2 and C are used for the radical control materials. The result indicates that when aluminum is used as the radical control material, there is no reaction with fluorine so that the fluorine atom density is large and that when carbon is used, the fluorine atom density is reduced to one-half. This means that a conversion reaction is considered to have occurred in which the carbon as the radical control material reacts with fluorine in the plasma to increase CF2. It is also found that during this process CF1 and CF2 have also increased. The fact that the use of SiO2 as the radical control material has resulted in reductions in CF1, CF2 and CF3 indicates that chemical reactions have occurred between CF1, CF2 and CF3 and the radical control material, SiO2. In this way, by placing in the plasma region a radical control material that reacts with reactive species in the plasma to produce volatile products, it is possible to transform the radical composition in the plasma. The use of silicon and silicon carbide for the radical control material, too, is found to cause chemical reactions that generate volatile products such as SiF2, thereby reducing fluorine in the plasma.
It was also found that the transforming of reactive species into a single selected species can be promoted by installing a voltage application system on the radical control material and applying a voltage to the radical control material during the sample etching. FIG. 3 shows densities of radicals, CF1, CF2 and CF3, measured by applying a negative DC voltage to carbon, the radical control material, in a CF4 gas plasma. It was found that as the applied voltage increases, CF3 decreases and CF2 increases. This phenomenon results from an ion-assisted reaction on the radical control material. Because of this phenomenon, fluorine in the plasma is transformed into CF2 and the plasma containing a single reactive species enables selective etching, in which reaction products on the silicon oxide film evaporate allowing the etching of the silicon oxide film to continue, whereas residual materials on the silicon nitride film stop the etching. This eliminates a problem of shoulder etching of the nitride film that would occur due to reduced selectivity. While the conventional etching balances carbon and fluorine, this invention is characterized by the use of CF2, which has a lower sticking parameter for sidewalls of the features being etched than that of carbon. This has been found to suppress the micro-loading and inverted micro-loading phenomena.
When etching samples having materials with largely differing etch rates, the following steps are taken. For the radical control material we use a compound which includes the same elements as those of the materials to be etched or at least one of the same elements as those of the etched materials. Depending on the material to be etched, the voltage applied to the radical control material is controlled according to the etching time or by monitoring the consumption or release of a certain kind of radical from the radical control material. This process is found to minimize localized abnormal deformations that would occur between different materials.
The problem of local deformations between different materials during metal etching can be solved by minimizing variations of the etchant. That is, this problem was able to be eliminated by using a radical control material having the same components as the material being etched and controlling the density of radicals according to the etching time or the monitoring of the result of radicals.
The time-dependent change of etching characteristic can be minimized by removing deposits from the surface that is in contact with the plasma. That is, by arranging the radical control material so as to enclose the plasma and then applying a voltage from outside during the etching of the sample, it was possible to automatically remove deposits adhering to the plasma contact surface. It is also noted that application of voltage during the sample etching process has improved the through-put. FIG. 5 shows the result of measurements by a step meter of a layer deposited when an arbitrary voltage was applied to an aluminum plate placed on a surface contacting a C4F8 plasma having a pressure of 1.5 mTorr and a microwave power of 200 W. The deposited film thickness depends on the density of the plasma, and deposits can be prevented from adhering to the surface by applying an appropriate voltage to the boundary surface with plasma, thus minimizing time-dependent variations in the etching characteristics.